1. Technical Field
Exemplary aspects of the present disclosure generally relate to a pressing device that presses a pressing member against a target such as an image bearer, and more particularly, to a transfer device including the pressing device and an image forming apparatus including the transfer device.
2. Description of the Related Art
Known electrophotographic image forming apparatuses form a latent image on a uniformly charged surface of an image bearer by writing optically an image based on image information. The latent image is developed with toner by a developing device to form a visible image, known as a toner image. Subsequently, the toner image is transferred onto a recording medium (recording material), and is fixed thereon.
In the image forming apparatuses of this kind, the toner image is formed on a drum-shaped photoconductor serving as the image bearer through a known electrophotographic process. An intermediate transfer belt formed into an endless loop contacts the photoconductor to form a so-called primary transfer nip therebetween. In the primary transfer nip, the toner image on the photoconductor is primarily transferred onto the intermediate transfer belt. A secondary transfer roller serving as a nip forming member contacts the intermediate transfer belt to form a so-called secondary transfer nip. A secondary-transfer opposing roller is disposed inside the looped intermediate transfer belt and opposite to the secondary transfer roller with the intermediate transfer belt interposed therebetween.
While the secondary-transfer opposing roller disposed inside the loop of the intermediate transfer belt is grounded, a secondary transfer bias (voltage) is applied to the secondary transfer roller disposed outside the loop. With this configuration, a secondary transfer electric field is formed between the secondary-transfer opposing roller and the secondary transfer roller so that the toner image moves electrostatically from the secondary-transfer opposing roller side to the secondary transfer roller side. A recording medium is fed to the secondary transfer nip in appropriate timing such that the recording medium is aligned with the toner image formed on the intermediate transfer belt. Due to the secondary transfer electrical field and a nip pressure in the secondary transfer nip, the toner image on the intermediate transfer belt is secondarily transferred onto the recording medium.
In recent years, a variety of recording media such Japanese paper known as “Washi” have come on market. Such recording media have a coarse surface through embossing process. A pattern of light and dark according to the surface condition of the recording medium appears in an output image. More specifically, toner does not transfer well to such embossed surfaces, in particular, recessed portions of the surface. This inadequate transfer of the toner appears as a pattern of light and dark patches in the resulting output image.
In order to prevent such a pattern of light and dark patches, the intermediate transfer belt is formed of an elastic belt including an elastic layer so that a higher nip pressure can be applied at the secondary transfer nip when transferring an image onto the recording medium having a coarse surface than when using a smooth recording medium. With a higher nip pressure, the surface of the intermediate transfer belt including the elastic layer can deform and thus fit the embossed surface of the recording medium. Accordingly, the transferability at the recessed portion of the recording medium surface is enhanced.
In order to enable the secondary transfer roller to contact the intermediate transfer belt, generally, the image forming apparatus includes a pressing device including a pressing member and a biasing member such as a spring. The pressing member supports both ends of the secondary transfer roller in an axial direction thereof. The biasing member applies the pressing member a biasing force. With this configuration, the biasing member presses the pressing member against the intermediate transfer belt, thereby enabling the secondary transfer roller supported by the pressing member to contact the intermediate transfer belt.
Although the biasing member with a relatively large biasing force can increase the nip pressure at the secondary transfer nip, the pressing member needs to be strong enough to accommodate such a large biasing force.